Grain-oriented electrical steel sheet

ABSTRACT

A grain-oriented electrical steel sheet includes a steel layer and an insulation coating arranged in directly contact with the steel layer thereon. The steel layer includes, as a chemical composition, by mass %, 2.9 to 4.0% of Si, 2.0 to 4.0% of Mn, 0 to 0.20% of Sn, and 0 to 0.20% of Sb. In the steel layer, a silicon content and a manganese content expressed in mass % satisfy 1.2%≤Si−0.5×Mn≤2.0%, and a tin content and an antimony content expressed in mass % satisfy 0.005%≤Sn+Sb≤0.20%.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of copending U.S. application Ser. No.15/502,682, filed on Feb. 8, 2017, which is the National Phase under 35U.S.C. § 371 of International Application No. PCT/JP2015/072896, filedon Aug. 13, 2015, which claims the benefit under 35 U.S.C. § 119(a) toPatent Application No. 2014-177136, filed in Japan on Sep. 1, 2014, allof which are hereby expressly incorporated by reference into the presentapplication.

TECHNICAL FIELD

The present invention relates to a grain-oriented electrical steel sheetsuitable for the segment core of rotating machines such as motor orgenerator, the lamination core of stationary apparatus such astransformer or reactor, or the like. In particular, the presentinvention relates to a grain-oriented electrical steel sheet in whichthe high-frequency magnetic properties along L-direction are almost thesame as those of conventional one and the high-frequency magneticproperties along C-direction are preferably improved.

Priority is claimed on Japanese Patent Application No. 2014-177136,filed Sep. 1, 2014, the content of which is incorporated herein byreference.

BACKGROUND ART

The grain-oriented electrical steel sheet has the excellent magneticproperties along rolling direction, because the crystal orientationthereof highly aligns in the {110}<001> orientation called Gossorientation. Thus, the grain-oriented electrical steel sheet has beenwidely applied to the iron core materials such as transformer,generator, or motor. In recent years, since power electronics havedeveloped, the high-frequency range over conventional commercialfrequency range has been increasingly utilized as the drive frequencyfor the rotating machines such as motor or generator, the stationaryapparatus such as transformer or reactor, or the like. Thus, it iseagerly anticipated to further improve the core loss characteristics inhigh-frequency range for the grain-oriented electrical steel sheet.

In addition, in case of using the drive motor which employs the segmentcore for hybrid vehicles (HEV), electric vehicles (EV), or the like, theexcellent core loss characteristics in high-frequency range are desiredin both directions of teeth and back yoke of the iron core. Thus, inaddition to the core loss characteristics along the rolling direction(L-direction) in high-frequency range, it is eagerly anticipated tofurther improve the core loss characteristics along the transversedirection (C-direction) perpendicular to the rolling direction inhigh-frequency range for the grain-oriented electrical steel sheet.Specifically, in the grain-oriented electrical steel sheet, in additionto the high-frequency core loss along L-direction (L-direction coreloss), it is required to be excellent in the average of thehigh-frequency core losses along L-direction and C-direction (L&Caverage core loss).

Herein, the segment core indicates the component included in the statorarranged on the periphery of rotor of motor. The segment core is punchedfrom the grain-oriented electrical steel sheet so that the radialdirection of motor rotational axis is substantially parallel to therolling direction (L-direction) of the electrical steel sheet, and thecircumferential direction of motor rotational axis is substantiallyparallel to the direction (C-direction) perpendicular to the rollingdirection of the electrical steel sheet. Specifically, in the segmentcore, the teeth which is important for magnetic properties in general issubstantially parallel to the rolling direction of the electrical steelsheet, and the back yoke is substantially parallel to the directionperpendicular to the rolling direction. In case of the stator in whichthe back yoke is important for magnetic properties, the segment core maybe punched so that the back yoke is substantially parallel to therolling direction of the electrical steel sheet.

Also, the core loss indicates the energy loss caused by theinterconversion of electrical energy and magnetic energy. It ispreferable that the value of core loss is low. The core loss of thegrain-oriented electrical steel sheet is able to be broken down into twoelements of hysteresis loss and eddy current loss. In particular, inorder to reduce the high-frequency core loss, it is effective to reducethe eddy current loss by controlling the steel to be highly alloyed andby increasing the electrical resistance of steel. Although it ispossible to reduce the eddy current loss by controlling the electricalsteel sheet to be thin, it is inevitable to increase the production costin order to control the electrical steel sheet to be thin due to adecrease in efficiency of cold rolling, annealing, or the like.

In conventional grain-oriented electrical steel sheets, the magneticanisotropy is obtained by the texture control, and thereby, the magneticproperties along the L-direction are significantly excellent. However,the magnetic properties along the C-direction thereof are markedlyinsufficient. Thus, it is unsuitable to apply the conventionalgrain-oriented electrical steel sheet to the segment core in which it isrequired to be balance the L&C average core loss with the L-directioncore loss.

In addition, as explained above, in order to reduce the high-frequencycore loss, it is effective to control the steel to be highly alloyed.However, when Si which is the main alloying element of the electricalsteel sheet is added in surplus as compared with that of conventionalone, the steel embrittles, and thereby, the cold rolling is hardlyconducted. Also, Al is the alloying element which may not embrittle thesteel as compared with Si. However, when Al is added in surplus to thesteel, it is difficult to control the dispersion state of the inhibitorAlN which importantly functions for controlling the crystal orientationin secondary recrystallization.

Patent Document 1 discloses the method for producing the electricalsteel sheet excellent in the balance between the magnetic properties inL-direction and C-direction. In the method thereof, the steel slabincluding 2.0 to 4.0% of Si, 0.5% or less of Mn, 0.003 to 0.020% of sol.Al, or the like is subjected to hot-rolling, hot-band annealing,cold-rolling twice with intermediate annealing, primaryrecrystallization annealing, and secondary recrystallization annealing.

Patent Document 2 discloses the method for producing the electricalsteel sheet excellent in the balance between the magnetic properties inL-direction and C-direction. In the method thereof, the steel slabincluding 2.5 to 4.0% of Si, 2.0 to 4.0% of Mn, 0.003 to 0.030% ofacid-soluble Al, or the like is subjected to hot-rolling, optionallyhot-band annealing, cold-rolling, primary recrystallization annealing,and secondary recrystallization annealing.

RELATED ART DOCUMENTS Patent Documents

[Patent Document 1] Japanese Unexamined Patent Application, FirstPublication No. H11-350032

[Patent Document 2] Japanese Unexamined Patent Application, FirstPublication No. H07-18334

SUMMARY OF INVENTION Technical Problem to be Solved

In the electrical steel sheet produced by the method disclosed in PatentDocument 1, the amount of alloying elements is insufficient, andtherefore, the high-frequency core loss is not sufficiently reduced.

In the electrical steel sheet according to Patent Document 2, PatentDocument 2 only considers the core loss in commercial frequency range of50 to 60 Hz, and therefore, the high-frequency core loss is notsufficiently reduced. Also, in the producing method according to PatentDocument 2, the secondary recrystallization tends to be unstable, andtherefore, the electrical steel sheet is not stably produced.

The present invention has been made in consideration of the abovementioned problems. An object of the present invention is to provide thegrain-oriented electrical steel sheet excellent in both thehigh-frequency magnetic properties in L-direction and the average ofhigh-frequency magnetic properties in L-direction and C-direction.

Solution to Problem

The present inventors found that, by including a large amount of Mnwhich is difficult to embrittle the steel in common with Al depending onSi content in the steel, by controlling a total amount of Sn and Sb inthe steel, and by optimally controlling the production conditions, it ispossible to obtain the electrical steel sheet in which thehigh-frequency magnetic properties along L-direction are almost the sameas those of conventional one and the high-frequency magnetic propertiesalong C-direction are preferably improved.

In the grain-oriented electrical steel sheet according to an aspect ofthe present invention, the thickness of sheet is 0.1 to 0.40 mm, and themagnetic flux density B8 along the rolling direction is 1.60 to 1.77 T.When the magnetic flux density B8 along the rolling direction is 1.60 to1.77 T, the balance between the L-direction core loss and the L&Caverage core loss is preferably controlled. The present inventors foundthat, when the magnetic flux density B8 along the rolling direction isless than 1.60 T, the L-direction core loss is insufficient. Also, thepresent inventors found that, when the magnetic flux density B8 alongthe rolling direction is more than 1.77 T, the L-direction core loss issufficient, however the C-direction core loss deteriorates, and as aresult, the L&C average core loss drastically deteriorates.

An aspect of the present invention employs the following.

(1) A grain-oriented electrical steel sheet according to an aspect ofthe present invention includes a steel layer and an insulation coatingarranged on the steel layer, wherein: the steel layer includes, as achemical composition, by mass %, 0.0003 to 0.005% of C, 2.9 to 4.0% ofSi, 2.0 to 4.0% of Mn, 0.003 to 0.018% of sol. Al, 0.005% or less of S,0 to 0.20% of Sn, 0 to 0.20% of Sb, and a balance consisting of Fe andimpurities; a silicon content and a manganese content expressed in mass% in the chemical composition of the steel layer satisfy1.2%≤Si−0.5×Mn≤2.0%; a tin content and an antimony content expressed inmass % in the chemical composition of the steel layer satisfy0.005%≤Sn+Sb≤0.20%; and the insulation coating is arranged in directlycontact with the steel layer.

(2) In the grain-oriented electrical steel sheet according to (1), thesteel layer may include, as the chemical composition, by mass %, 0.004to 0.20% of Sn, and 0.001 to 0.20% of Sb.

(3) A method of producing the grain-oriented electrical steel sheetaccording to (1) or (2) includes a casting process, a hot-rollingprocess, a cold-rolling process, a primary recrystallization annealingprocess, an annealing separator coating process, a secondaryrecrystallization annealing process, and an insulation coating formationprocess, wherein: in the casting process, a steel is cast so that thesteel includes, as a chemical composition, by mass %, 0.0003 to 0.005%of C, 2.9 to 4.0% of Si, 2.0 to 4.0% of Mn, 0.003 to 0.018% of sol. Al,0.001 to 0.01% of N, 0.005% or less of S, 0 to 0.20% of Sn, 0 to 0.20%of Sb, and a balance consisting of Fe and impurities, a silicon contentand a manganese content expressed in mass % in the chemical compositionsatisfy 1.2%≤Si−0.5×Mn≤2.0%, and a tin content and an antimony contentexpressed in mass % in the chemical composition satisfy0.005%≤Sn+Sb≤0.20%; in the primary recrystallization annealing process,a primary recrystallization annealing is conducted for the steel underconditions such that a heating rate in a temperature elevating stage is100° C./second to 5000° C./second on average, an atmosphere in thetemperature elevating stage is 10 to 100 vol % of H2 and H2+N2=100 vol%, a temperature in a holding stage is 800 to 1000° C., a time in theholding stage is 5 seconds to 10 minutes, an atmosphere in the holdingstage is 10 to 100 vol % of H2, H2+N2=100 vol %, and a dew point is −10°C. or lower; in the annealing separator coating process, an annealingseparator including an alumina as a main component is only applied onthe steel; and in the secondary recrystallization annealing process, asecondary recrystallization annealing is conducted for the steel underconditions such that an atmosphere in a temperature elevating stage is 0to 80 vol % of N2 and H2 +N2=100 vol %, a dew point in a temperaturerange of 500° C. or higher in the temperature elevating stage is 0° C.or lower, a temperature in a holding stage is 850 to 1000° C., a time inthe holding stage is 4 to 100 hours, an atmosphere in the holding stageis 0 to 80 vol % of N2, H2+N2=100 vol %, and a dew point is 0° C. orlower.

(4) In the method of producing the grain-oriented electrical steel sheetaccording to (3), in the casting process, the steel may include, as thechemical composition, by mass %, 0.004 to 0.20% of Sn, and 0.001 to0.20% of Sb.

(5) In the method of producing the grain-oriented electrical steel sheetaccording to (3) or (4), in the secondary recrystallization annealingprocess, the steel is heated to the temperature in the holding stage bya constant heating rate in the temperature elevating stage.

Effects of Invention

According to the above aspects of the present invention, it is possibleto provide the grain-oriented electrical steel sheet excellent in boththe high-frequency magnetic properties in L-direction and the average ofhigh-frequency magnetic properties in L-direction and C-direction.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional illustration of a grain-oriented electricalsteel sheet according to an embodiment of the present invention.

FIG. 2 is a cross-sectional illustration of a conventionalgrain-oriented electrical steel sheet.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Hereinafter, a preferable embodiment of the present invention will bedescribed in detail. However, the present invention is not limited onlyto the configuration which is disclosed in the embodiment, and variousmodifications are possible without departing from the aspect of thepresent invention. In addition, the limitation range as described belowincludes a lower limit and an upper limit thereof. However, the valueexpressed by “more than” or “less than” is not include in the limitationrange.

Hereinafter, the grain-oriented electrical steel sheet according to theembodiment will be described in detail.

The present inventors thoroughly review the chemical composition of thesteel and the production conditions, and thereby, succeed in obtainingthe grain-oriented electrical steel sheet excellent in the balancebetween the high-frequency core losses in L-direction and C-direction.

The grain-oriented electrical steel sheet according to the embodimentincludes a steel layer (base steel) and an insulation coating arrangedon the steel layer, wherein: the steel layer includes, as a chemicalcomposition, by mass %, 0.0003 to 0.005% of C, 2.9 to 4.0% of Si, 2.0 to4.0% of Mn, 0.003 to 0.018% of sol. Al, 0.005% or less of S, 0 to 0.20%of Sn, 0 to 0.20% of Sb, and a balance consisting of Fe and impurities;a silicon content and a manganese content expressed in mass % in thechemical composition of the steel layer satisfy 1.2%≤Si−0.5×Mn≤2.0%; atin content and an antimony content expressed in mass % in the chemicalcomposition of the steel layer satisfy 0.005%≤Sn+Sb≤0.20%; and theinsulation coating is arranged in directly contact with the steel layer.In addition, it is preferable that the thickness of the grain-orientedelectrical steel sheet is 0.1 to 0.40 mm and the magnetic flux densityB8 along the rolling direction of the grain-oriented electrical steelsheet is 1.60 to 1.77 T.

FIG. 1 shows the grain-oriented electrical steel sheet according to theembodiment in case of viewing the cross section whose cutting directionis parallel to the thickness direction. In the grain-oriented electricalsteel sheet according to the embodiment, the insulation coating 2 isarranged in directly contact with the steel layer 1 (base steel). FIG. 2shows the conventional grain-oriented electrical steel sheet in case ofviewing the cross section whose cutting direction is parallel to thethickness direction. In the conventional grain-oriented electrical steelsheet, the glass film 3 (forsterite film) is arranged on the steel layer1 (base steel), and the insulation coating 2 is arranged on the glassfilm 3 (forsterite film).

(1) Chemical Composition of Steel Layer (Base Steel)

The chemical composition of steel layer of the grain-oriented electricalsteel sheet according to the embodiment will be described in detail.Hereinafter, “%” of the amount of respective elements as described belowexpresses “mass %” unless otherwise mentioned.

In the chemical composition of steel layer of the grain-orientedelectrical steel sheet according to the embodiment, base elements are C,Si, Mn, and Al.

C: 0.0003 to 0.005%

C (carbon) is the base element in the steel but the element which causesthe deterioration of core loss. Thus, it is preferable that C content isas small as possible. In the grain-oriented electrical steel sheetaccording to the embodiment, the upper limit of C content is to be0.005%. When the C content is more than 0.005%, the core loss of thegrain-oriented electrical steel sheet deteriorates, and thus, theexcellent magnetic properties are not obtained. The upper limit of Ccontent is preferably 0.004% and more preferably 0.003%. On the otherhand, although the lower limit of C content is not particularly limited,the lower limit is to be 0.0003%. Since the production cost for steelmaking is excessive, it is industrially difficult to control the Ccontent to be less than 0.0003%.

Si: 2.9 to 4.0%

Si (silicon) has the effect in increasing the electrical resistance ofsteel, reducing the eddy current loss, and thereby, improving thehigh-frequency core loss. In order to effectively obtain the effect, thelower limit of Si content is to be 2.9%. The lower limit of Si contentis preferably 3.0%. On the other hand, the upper limit of Si content isto be 4.0%. When the Si content is more than 4.0%, the workabilitydrastically deteriorates, and thus, it is difficult to conduct thecold-rolling. The upper limit of Si content is preferably 3.8%.

Mn: 2.0 to 4.0%

Mn (manganese) has the effect in increasing the electrical resistance ofsteel without the deterioration of the workability of steel, reducingthe eddy current loss, and thereby, improving the high-frequency coreloss. In order to effectively obtain the effect, the lower limit of Mncontent is to be 2.0%. When the Mn content is less than 2.0%. The effectin reducing the high-frequency core loss is insufficient. The lowerlimit of Mn content is preferably 2.2% and more preferably 2.6%. On theother hand, the upper limit of Mn content is to be 4.0%. When the Mncontent is more than 4.0%, the magnetic flux density drasticallydecreases. The upper limit of Mn content is preferably 3.8% and morepreferably 3.4%.

Si−0.5×Mn: 1.2 to 2.0%

In addition, in the embodiment, the Si content and the Mn content areregulated in connection with each other. In order to let the secondaryrecrystallization proceed stably, it is necessary to control themicrostructure of hot-rolled steel sheet to be uniform and fine.Therefore, in the embodiment, the transformation between a (ferrite) andγ (austenite) is utilized. In the conventional grain-oriented electricalsteel sheet, C which is an austenite-forming element is contained at thestage of the hot-rolled steel sheet. However, in the electrical steelsheet according to the embodiment, the C content is low at the stage ofthe hot-rolled steel sheet. Thus, in the electrical steel sheetaccording to the embodiment, the α-γ transformation is mainly affectedby the balance between the amount of Si which is a ferrite-formingelement and the amount of Mn which is an austenite-forming element. Itis necessary to regulate the Si content and the Mn content in connectionwith each other.

Specifically, the upper limit of the value calculated by “(Sicontent)−0.5×(Mn content)” is to be 2.0%. When the value is more than2.0%, the α-γ transformation does not occur sufficiently, themicrostructure of hot-rolled steel sheet is not controlled to be uniformand fine, and the secondary recrystallization becomes unstable. Theupper limit of “Si−0.5×Mn” is preferably 1.8% and more preferably 1.75%.On the other hand, although the lower limit of “Si−0.5×Mn” is notparticularly limited, the lower limit is to be 1.2%. In order to let thesecondary recrystallization proceed stably, the lower limit of“Si−0.5×Mn” is preferably 1.6%. When the Si content and the Mn contentsatisfy the above condition, it is possible to obtain the grain-orientedelectrical steel sheet excellent in the balance between thehigh-frequency core losses in L-direction and C-direction.

sol. Al: 0.003 to 0.018%

Sol. Al (acid-soluble aluminum) forms the inhibitor which importantlyfunctions for controlling the crystal orientation in secondaryrecrystallization. The inhibitor is the nitrides as the precipitates,for example, the (Al, Si, Mn) composite nitrides. In the embodiment, thelower limit of sol. Al content is to be 0.003%. When the sol. Al contentis less than 0.003%, the inhibitor's effect is not sufficientlyobtained. On the other hand, the upper limit of sol. Al content is to be0.018%. When the sol. Al content is more than 0.018%, the dispersionstate of the nitrides is unfavorable, and thus, the secondaryrecrystallization does not proceed stably. The upper limit of sol. Alcontent is preferably 0.016%.

The steel layer of the grain-oriented electrical steel sheet accordingto the embodiment includes, as the chemical composition, the impurities.Herein, “impurities” represent elements which are contaminated duringindustrial production of the steel from ores and scrap that are used asa raw material of the steel, or from environment of a productionprocess. Among the impurities, it is preferable that S is limited asfollows in order to sufficiently obtain the effects of the embodiment.Moreover, since it is preferable that the amount of respectiveimpurities is low, a lower limit does not need to be limited, and thelower limit of the respective impurities may be 0%.

S: 0.005% or less

S (sulfur) is the impurity. S forms MnS by bonding to Mn in the steel,and thus, the magnetic properties deteriorate. Therefore, the S contentis limited to 0.005% or less. The upper limit of S content is preferably0.004% and more preferably 0.003%.

The steel layer of the grain-oriented electrical steel sheet accordingto the embodiment includes the above mentioned base elements and thebalance contains Fe and the above mentioned impurities. However, thesteel layer of the grain-oriented electrical steel sheet according tothe embodiment includes at least one of Sn or Sb in substitution for apart of Fe which is the balance.

Sn: 0 to 0.20%

Sb: 0 to 0.20%

Sn+Sb: 0.005 to 0.20%

Sn (tin) and Sb (antimony) are the elements which let the secondaryrecrystallization proceed stably and which let the high-frequency coreloss be reduced by refining the secondary recrystallized grains. Inorder to obtain the effect, the Sn content is to be 0 to 0.20%, the Sbcontent is to be 0 to 0.20%, and the total amount of Sn and Sb is to be0.005 to 0.20%. When one of Sn or Sb is contained in the steel layer,the other is not necessarily contained in the steel layer. Thus, thelower limit of each amount of Sn and Sb may be 0%. However, the lowerlimit of the total amount of Sn and Sb is to be 0.005%. The lower limitof the total amount of Sn and Sb is preferably 0.01%. On the other hand,the upper limit of the total amount of Sn and Sb is to be 0.20%. Whenthe upper limit of the total amount of Sn and Sb is more than 0.20%, theabove effect is saturated. The upper limit of the total amount of Sn andSb is preferably 0.15% and more preferably 0.13%.

As described above, at least one of Sn or Sb may be included in thesteel layer. However, it is preferable that both Sn and Sb aresimultaneously included in the steel layer. For example, it ispreferable that the lower limit of Sn content is 0.004% and the lowerlimit of Sb content is 0.001%. Specifically, it is preferable that thesteel layer of the grain-oriented electrical steel sheet according tothe embodiment includes Mn, Sn, and Sb at the same time. When thecondition is satisfied, the average of the high-frequency core lossesalong L-direction and C-direction is preferably improved.

In the grain-oriented electrical steel sheet according to theembodiment, the Mn content and the Sn+Sb total content aresimultaneously increased as the chemical composition of the steel layer.By increasing the Mn content, it is possible to increase the electricalresistance of steel, to reduce the eddy current loss, and as a result,to improve the average of the high-frequency magnetic properties (corelosses) along L-direction and C-direction. Also, by increasing the Sn+Sb total content, it is possible to refine the secondary recrystallizedgrains, to reduce the excess eddy current loss, and as a result, toimprove the average of the high-frequency magnetic properties (corelosses) along L-direction and C-direction.

On the other hand, in the conventional grain-oriented electrical steelsheet, it is not necessarily easy to increase the Mn content and theSn+Sb total content at the same time. In particular, when the Mn contentis increased, it is not necessarily easy to include Sn and Sb at thesame time. In the conventional grain-oriented electrical steel sheet,including Mn, Sn, and Sb at the same time causes the problem such thatthe adhesion of insulation coating drastically deteriorates. The presentinventors assume that the above problem is derived from the excessiveoxidation which occurs in the vicinity of the surface of steel layerduring the primary recrystallization annealing and the secondaryrecrystallization annealing.

In the conventional grain-oriented electrical steel sheet, in general,the steel slab includes C whose content stabilizes the austenite atannealing temperature and hot rolling temperature, the heating forprimary recrystallization annealing is conducted by heating rate ofslower than 100° C./second, the primary recrystallization annealing isconducted in moist atmosphere (decarburizing atmosphere), themagnesia-based separator is used as an annealing separator, and thesecondary recrystallization annealing is conducted. In the primaryrecrystallization annealing, since the atmosphere is moist(decarburizing atmosphere), the oxidation in addition to decarburizationis promoted in the steel sheet (steel layer). Also, since MgO in theannealing separator which is slurried for applying to the steel sheet(steel layer) is changed to Mg(OH)2, the magnesia annealing separatortends to oxidize the steel sheet (steel layer). Also, in the secondaryrecrystallization annealing, the magnesia annealing separator and theoxide layer (silica) at the surface of steel sheet (steel layer) arechemically reacted, and thus, the glass film (forsterite film) isformed.

In case of applying the above conventional producing method to the steelsheet (steel layer) where the Mn content and the Sn +Sb total contentare simultaneously increased, the vicinity of the surface of steel layermay be excessively oxidized due to the peculiar chemical composition ofsteel. As a result, the problem such that the adhesion of insulationcoating drastically deteriorates may occur. In the embodiment, byoptimally controlling the chemical composition of steel layer and theproduction conditions, it is possible to increase the Mn content and theSn +Sb total content at the same time. In particular, in addition toincreasing the Mn content, it is possible to increase Sn and Sb at thesame time. Although the detail of production conditions will bedescribed below, in the embodiment, the C content in steel slab iscontrolled to be low, the rapid heating as compared with conventionalheating is conducted in the temperature elevating stage of primaryrecrystallization annealing, the primary recrystallization annealing isconducted in dry atmosphere (non-decarburizing atmosphere), thealumina-based separator is used as the annealing separator, and thesecondary recrystallization annealing is conducted in dry atmosphere.

The grain-oriented electrical steel sheet produced by the specificproduction conditions according to the embodiment does not include theglass film (forsterite film) on the steel layer, because the primaryrecrystallization annealing is conducted in dry atmosphere(non-decarburizing atmosphere), the alumina-based separator is used asthe annealing separator, and the secondary recrystallization annealingis conducted in dry atmosphere. Specifically, in the grain-orientedelectrical steel sheet according to the embodiment, the insulationcoating is arranged in directly contact with the steel layer.

Even when the Mn content and the Sn +Sb total content are simultaneouslyincreased in the steel layer, in particular, even when Sn and Sb areincluded at the same time in addition to increasing the Mn content, itis possible to suppress the excessive oxidation in the vicinity of thesurface of steel layer by applying the specific production conditionsaccording to the embodiment. Thus, it is possible to suppress thedecrease in the adhesion of insulation coating. As a result, it ispossible to preferably improve the average of the high-frequencymagnetic properties (core losses) along L-direction and C-direction ascompared with those of conventional one.

In addition, in the grain-oriented electrical steel sheet according tothe embodiment, even when the Sn +Sb total content is increased inaddition to increasing the Mn content, it is possible to preferablysuppress the decrease in the punchability.

Sn and Sb are the elements which tend to embrittle the steel. Withrespect to the Si steel (steel layer) whose workability is essentiallypoor, when the Sn+Sb total content is increased in addition toexcessively increasing the Mn content although Mn is difficult toembrittle the steel, the workability of steel may drasticallydeteriorate.

Although the details are unclear, when the Si steel (steel layer)includes the large amount of Mn, Mn oxides may be formed in addition toSi oxides in the vicinity of the surface of steel layer, Sn and Sb maybe segregated in the vicinity of the Si oxides and the Mn oxides, andthereby, the punchability may deteriorate. Thus, in the conventionalgrain-oriented electrical steel sheet, it is not necessarily easy toincrease the Mn content and the Sn+Sb total content at the same time. Inparticular, when the Mn content is increased, it is not necessarily easyto include Sn and Sb at the same time.

In the specific production conditions according to the embodiment, theoxidation of Mn is suppressed in the vicinity of the surface of steellayer during the primary recrystallization annealing, and the glass filmis not formed during the secondary recrystallization annealing. Sincethe oxidation of Mn is suppressed and the glass film is not formed, theoxides are not excessive in the vicinity of the surface of steel layer.In particular, in addition to the specific production conditionsaccording to the embodiment, when the steel simultaneously includes Snand Sb and when the alumina-based separator is used as the annealingseparator, the oxide layer further thins. Also, the segregation of Snand Sb is suppressed. Therefore, the deterioration of punchability ispreferably suppressed. The reason seems that the brittle fracture whoseorigin is in the vicinity of the surface (whose origin is in theinterface between the steel layer and the insulation coating) issuppressed by thinning the oxide layer in the vicinity of the surface ofthe steel layer.

Specifically, in the steel layer of the grain-oriented electrical steelsheet according to the embodiment (the whole steel layer without theinsulation coating), the amount of O (oxygen) is preferably 0.03% (300ppm) or less in mass %. Also, when the surface area of steel layer isthe area within 10 μm in depth toward the steel layer from the interfacebetween the steel layer and the insulation coating, the O content ispreferably less than 0.01% (100 ppm) in mass % in the body area which isthe area except for the surface area in the steel layer. When the Ocontent in the whole steel layer is 0.03% (300 ppm) or less, thedeterioration of punchability is preferably suppressed. The O content inthe whole steel layer is preferably 0.02% (200 ppm) or less and morepreferably 0.01% (100 ppm) or less. In addition, although the lowerlimit of O content in the whole steel layer is not particularly limited,the lower limit may be 0.001% (10 ppm). The O content in the steel layermay be measured by, for example, the non-dispersive infrared absorptionmethod after fusion in a current of inert gas.

The steel layer of the grain-oriented electrical steel sheet accordingto the embodiment may further include the optional element in additionto the above explained elements. For example, the steel layer mayfurther include, as the optional element, at least one selected from thegroup consisting of N, P, Ni, Cr, Cu, and Mo in substitution for a partof Fe which is the balance. The optional elements may be included asnecessary. Thus, a lower limit of the respective optional elements doesnot need to be limited, and the lower limit may be 0%. Moreover, even ifthe optional elements may be included as impurities, the above mentionedeffects are not affected.

N: 0 to 0.01%

N (nitrogen) forms the nitrides which act as the inhibitor. Thus, thecontent in the steel slab is preferably 0.0010% or more. However, whenthe large amount of N remains in the steel layer of the grain-orientedelectrical steel sheet as final products, the magnetic properties may benegatively influenced. Thus, the upper limit of N content is preferably0.0100% and more preferably 0.0050%.

P: 0 to 0.15%

P (phosphorus) has the effect in reducing the eddy current loss byincreasing the electrical resistance of steel. Thus, the P content maybe 0 to 0.15%. The lower limit of P content is preferably 0.0001%.

Ni: 0 to 0.3%

Ni (nickel) has the effect in reducing the eddy current loss byincreasing the electrical resistance of steel and in improving themagnetic flux density. Thus, the Ni content may be 0 to 0.3%. The lowerlimit of Ni content is preferably 0.0001%.

Cr: 0 to 0.3%

Cr (chromium) has the effect in reducing the eddy current loss byincreasing the electrical resistance of steel. Thus, the Cr content maybe 0 to 0.3%. The lower limit of Cr content is preferably 0.0001%.

Cu: 0 to 0.3%

Cu (copper) has the effect in reducing the eddy current loss byincreasing the electrical resistance of steel. Thus, the Cu content maybe 0 to 0.3%. The lower limit of Cu content is preferably 0.0001%.

Mo: 0 to 0.3%

Mo (molybdenum) has the effect in reducing the eddy current loss byincreasing the electrical resistance of steel. Thus, the Mo content maybe 0 to 0.3%. The lower limit of Mo content is preferably 0.0001%.

The chemical composition of the steel layer as described above may bemeasured by typical analytical methods for the steel. For example, thechemical composition of the steel layer may be measured by using ICP-AES(Inductively Coupled Plasma-Atomic Emission Spectrometer: inductivelycoupled plasma emission spectroscopy spectrometry). Specifically,granular specimens are taken from center position of the steel layerafter removing the coating, chemical analysis is conducted under theconditions based on the predetermined working curve, and thereby, thechemical composition is identified. In addition, C and S may be measuredby the infrared absorption method after combustion, N may be measured bythe thermal conductometric method after fusion in a current of inertgas, and O may be measured by, for example, the non-dispersive infraredabsorption method after fusion in a current of inert gas.

(2) Thickness of Grain-Oriented Electrical Steel Sheet

Next, a preferable thickness of the grain-oriented electrical steelsheet according to the embodiment will be described.

In the grain-oriented electrical steel sheet according to theembodiment, the upper limit of thickness may be 0.40 mm. When thethickness is thicker than 0.40 mm, the eddy current loss may increaseand the high-frequency core loss may deteriorate. On the other hand,although the lower limit of thickness is not particularly limited, thelower limit may be 0.1 mm. When the thickness is thinner than 0.1 mm,the productivity is undesirably lowered.

(3) Magnetic Properties of Grain-Oriented Electrical Steel Sheet

Next, preferable magnetic properties of the grain-oriented electricalsteel sheet according to the embodiment will be described.

In the grain-oriented electrical steel sheet according to theembodiment, the lower limit of magnetic flux density B8 along therolling direction (L-direction) is preferably 1.60 T. When the magneticflux density B8 along the rolling direction is less than 1.60 T, boththe L-direction core loss and the L&C average core loss may deteriorate.The lower limit of magnetic flux density B8 along the rolling directionis preferably 1.62 T. On the other hand, the upper limit of magneticflux density B8 along the rolling direction is preferably 1.77 T. Whenthe magnetic flux density B8 along the rolling direction is more than1.77 T, the L-direction core loss is sufficient, however the C-directioncore loss deteriorates, and as a result, the L&C average core lossdrastically deteriorates. The upper limit of magnetic flux density B8along the rolling direction is preferably 1.76 T.

In addition, in the grain-oriented electrical steel sheet according tothe embodiment, the core loss W10/400 along L-direction is preferably13.0 W/kg or less. Also, the average of the core losses W10/400 alongL-direction and C-direction is preferably 14.5 W/kg or less. In the coreloss characteristics, since it is preferable that the value thereof islow, the lower limit thereof is not particularly limited. Also, the coreloss W10/400 along C-direction is preferably 1.0 to 2.0 times ascompared with the core loss W10/400 along L-direction. When the aboveconditions are satisfied, the average of the high-frequency core lossesalong L-direction and C-direction is preferably improved.

Herein, the magnetic properties such as the magnetic flux density andthe core loss may be measured by a known method, for example, theepstein test regulated by JIS C2550, the single sheet tester (SST)method regulated by JIS C 2556, or the like. Also, the magnetic fluxdensity B8 indicates the magnetic flux density under the magnetizingfield of 800 A/m, and the core loss W10/400 indicates the core lossunder conditions such that the maximum magnetic flux density is 1.0 Tand the frequency is 400 Hz.

Next, a method of producing the grain-oriented electrical steel sheetaccording to the embodiment will be described in detail.

The method of producing the grain-oriented electrical steel sheetaccording to the embodiment includes a casting process, a hot-rollingprocess, a cold-rolling process, a primary recrystallization annealingprocess, an annealing separator coating process, a secondaryrecrystallization annealing process, and an insulation coating formationprocess. As necessary, a hot-band annealing process may be includedafter the hot-rolling process and before the cold-rolling process. Inthe cold-rolling process, the cold-rolling may be conducted once ortwice or more with intermediate annealing.

Casting Process

In the casting process, a cast piece (slab) is cast so that the castpiece includes, as a chemical composition, by mass %, 0.0003 to 0.005%of C, 2.9 to 4.0% of Si, 2.0 to 4.0% of Mn, 0.003 to 0.018% of sol. Al,0.001 to 0.01% of N, 0.005% or less of S, 0 to 0.20% of Sn, 0 to 0.20%of Sb, and a balance consisting of Fe and impurities, a silicon contentand a manganese content expressed in mass % in the chemical compositionsatisfy 1.2%≤Si−0.5×Mn≤2.0%, and a tin content and an antimony contentexpressed in mass % in the chemical composition satisfy0.005%≤Sn+Sb≤0.20%. For example, the slab may be cast by the cast methodsuch as a continuous casting method, an ingot making method, or a thinslab casting method in general. In the case of the continuous casting,the steel may be subjected to the hot-rolling after the steel is cooledonce to a lower temperature (for example, room temperature) and isreheated, or the steel (cast slab) may be continuously subjected to thehot-rolling just after the steel is cast.

At least one of Sn or Sb may be included in the above cast piece (slab).However, it is preferable that both Sn and Sb are simultaneouslyincluded in the slab. For example, it is preferable that the lower limitof Sn content is 0.004% and the lower limit of Sb content is 0.001%.Specifically, it is preferable that the slab includes Mn, Sn, and Sb atthe same time in the casting process of the method of producing thegrain-oriented electrical steel sheet according to the embodiment.

Hot-Rolling Process

In the hot-rolling process, the slab after the casting process is heatedto 1050 to 1400° C., the hot-rolling is conducted for the above slab,and the hot-rolling is finished in the range of 700 to 950° C. In thehot-rolling process, the hot-rolling may be conducted so as to obtainthe hot-rolled steel sheet with the thickness of 1.8 to 3.5 mm.

Hot-Band Annealing Process

After the hot-rolling process, the hot-band annealing may be conductedas necessary. In the hot-band annealing process, for the hot-rolledsteel sheet after the hot-rolling process, the annealing may beconducted under conditions in 750 to 1200° C. for 10 seconds to 10minutes for continuous-annealing, and the annealing may be conductedunder conditions in 650 to 950° C. for 30 minutes to 24 hours forbox-annealing.

Cold-Rolling Process

In the cold-rolling process, the cold-rolling is conducted for thehot-rolled steel sheet after the hot-rolling process or the hot-bandannealed sheet after the hot-band annealing process. In the cold-rollingprocess, the cold-rolling may be conducted so as to obtain thecold-rolled steel sheet with the thickness of 0.1 to 0.4 mm. In casethat the cold-rolling may be conducted twice or more with intermediateannealing, the reduction of cold-rolling before the intermediateannealing may be 40 to 70%, and the reduction of final cold-rollingafter the intermediate annealing may be 40 to 90%. The intermediateannealing may be conducted under the same annealing conditions as thoseof the above hot-band annealing.

Primary Recrystallization Annealing Process

In the primary recrystallization annealing process, the primaryrecrystallization annealing is conducted for the cold-rolled steel sheetafter the cold-rolling process. In the primary recrystallizationannealing process, a rapid heating is conducted in a temperatureelevating stage. By conducting the rapid heating in the temperatureelevating stage of the primary recrystallization annealing process, itis possible to shorten the heating time, and as a result, to suppressthe surface oxidation during the temperature elevating stage. Inaddition, the holding is conducted in dry atmosphere (non-decarburizingatmosphere). Specifically, in the temperature elevating stage, theheating rate in the temperature elevating stage is 100° C./second to5000° C./second on average, the atmosphere in the temperature elevatingstage is 10 to 100 vol % of H2 and H2+N2=100 vol %, and the dew point ofthe atmosphere in the temperature elevating stage is preferably 0° C. orlower. In a holding stage, the temperature in the holding stage is 800to 1000° C., the time in the holding stage is 5 seconds to 10 minutes,the atmosphere in the holding stage is 10 to 100 vol % of H2, H2+N2=100vol %, and the dew point is −10° C. or lower. The heating rate in thetemperature elevating stage is preferably 100° C./second to 2000°C./second.

The atmosphere in the temperature elevating stage is preferably lessthan 50 vol % of H2 and more preferably less than 25 vol % of H2. Also,the atmosphere in the holding stage is preferably less than 50 vol % ofH2 and more preferably less than 25 vol % of H2. When the aboveconditions are satisfied, the average of the high-frequency core lossesalong L-direction and C-direction is preferably improved.

Annealing Separator Coating Process

In the annealing separator coating process, an annealing separatorincluding alumina (Al2O3) as main component is only applied on theprimary-recrystallized steel sheet after the primary recrystallizationannealing process. The annealing separator including magnesia (MgO) asmain component does not use, the magnesia being changed to hydroxideduring coating and thereby resulting in the large amount of O broughtin. By using the alumina-based separator, it is possible to suppress theexcessive oxidation in the vicinity of the surface of steel layer in thesecondary recrystallization annealing process.

Secondary Recrystallization Annealing Process

In the secondary recrystallization annealing process, the secondaryrecrystallization annealing is conducted for the separator-coated steelsheet after the annealing separator coating process. In the secondaryrecrystallization annealing process, an atmosphere in a temperatureelevating stage is controlled and the holding is conducted in dryatmosphere. Specifically, the atmosphere in the temperature elevatingstage is 0 to 80 vol % of N2 and H2+N2=100 vol %, the dew point in thetemperature range of 500° C. or higher in the temperature elevatingstage is 0° C. or lower, the temperature in the holding stage is 850 to1000° C., the time in the holding stage is 4 to 100 hours, theatmosphere in the holding stage is 0 to 80 vol % of N2, H2+N2=100 vol %,and the dew point is 0° C. or lower. The atmosphere in the holding stageis preferably 0 to 50 vol % of N2.

In the temperature elevating stage, the steel sheet may be heated to theabove temperature of 850 to 1000° C. in the holding stage by theconstant heating rate in substance (without the two stage annealing).The heating rate in the temperature of 800° C. or higher is preferably10 to 50° C./hour on average. The atmosphere in the temperatureelevating stage is preferably less than 30 vol % of N2 and morepreferably less than 20 vol % of N2. The atmosphere in the holding stageis preferably 100% of H2. When the above conditions are satisfied, theaverage of the high-frequency core losses along L-direction andC-direction is preferably improved.

Insulation Coating Formation Process

In the insulation coating formation process, the insulation coating isformed for the secondary recrystallized steel sheet after the secondaryrecrystallization annealing process. For example, the mixture of resinsuch as acrylic and inorganic material such as phosphate, the solutionfor insulation coating containing colloidal silica and phosphate, or thelike may be applied on the surface of steel sheet, and the heattreatment may be conducted in the temperature range of 250 to 400° C. incase that an organic is contained and the temperature range of 840 to920° C. in case that an inorganic is only contained.

The grain-oriented electrical steel sheet produced as mentioned aboveincludes a steel layer (base steel) and an insulation coating arrangedon the steel layer, wherein: the steel layer includes, as a chemicalcomposition, by mass %, 0.0003 to 0.005% of C, 2.9 to 4.0% of Si, 2.0 to4.0% of Mn, 0.003 to 0.018% of sol. Al, 0.005% or less of S, 0 to 0.20%of Sn, 0 to 0.20% of Sb, and a balance consisting of Fe and impurities;a silicon content and a manganese content expressed in mass % in thechemical composition of the steel layer satisfy 1.2%≤Si−0.5×Mn≤2.0%; atin content and an antimony content expressed in mass % in the chemicalcomposition of the steel layer satisfy 0.005%≤Sn+Sb≤0.20%; and theinsulation coating is arranged in directly contact with the steel layer.

In the grain-oriented electrical steel sheet produced by optimally andcomprehensively controlling the above production conditions, even whenthe Mn content and the Sn+Sb total content are simultaneously increasedin the steel layer, in particular, even when Sn and Sb are included atthe same time in addition to increasing the Mn content, it is possibleto suppress the excessive oxidation in the vicinity of the surface ofsteel layer. Thus, it is possible to suppress the decrease in theadhesion of insulation coating. Also, it is possible to preferablyimprove the average of the high-frequency magnetic properties (corelosses) along L-direction and C-direction.

EXAMPLES

Hereinafter, the effects of an aspect of the present invention will bedescribed in detail with reference to the following examples. However,the condition in the examples is an example condition employed toconfirm the operability and the effects of the present invention, sothat the present invention is not limited to the example condition. Thepresent invention can employ various types of conditions as long as theconditions do not depart from the scope of the present invention and canachieve the object of the present invention.

Example 1

The steel slabs whose chemical compositions were shown in Table 1 withthe balance consisting of Fe and impurities were heated to 1250° C. andthen were hot-rolled so that the thickness was 2.6 mm. The cold-rollingwas conducted so that the thickness was 1.2 mm, the intermediateannealing was conducted at 900° C. for 30 seconds, the final rolling wasconducted so that the final thickness was 0.30 mm, the primaryrecrystallization annealing was conducted at 920° C. for 15 seconds, theannealing separator was applied, the secondary recrystallizationannealing was conducted at the maximum temperature of 940° C., andthereafter, the insulation coating was formed.

In the primary recrystallization annealing process, the heating rate inthe temperature elevating stage was 400° C./second, the atmosphere inthe temperature elevating stage was 20% of H2 and 80% of N2, theatmosphere in the holding stage was 20% of H2 and 80% of N2, and the dewpoint in the holding stage was −20° C. The alumina-based separator wasused for the annealing separator. In the secondary recrystallizationannealing process, the heating rate in the temperature elevating stageof 800° C. or higher was 20° C./hour, the temperature was elevated to940° C. by the constant heating rate in substance, the atmosphere in thetemperature elevating stage was 85% of H2 and 15% of N2, the dew pointin the temperature range of 500° C. or higher in the temperatureelevating stage was −10° C., the time in the holding stage was 10 hours,the atmosphere in the holding stage was 100% of H2, and the dew point inthe holding stage was −30° C. In all steel sheets, the insulationcoating was arranged in directly contact with the steel layer, and theadhesion was sufficient.

TABLE 1 CHEMICAL COMPOSITION OF STEEL SLAB (mass %) Si − STEEL 0.5 ×TYPE C Si Mn S sol. Al N Mn Sn Sb NOTE A 0.002 3.00 2.63 0.002 0.0020.0038 1.69 Tr. Tr. COMPARATIVE EXAMPLE B 0.002 2.99 2.62 0.002 0.0090.0031 1.68 0.03 Tr. EXAMPLE C 0.002 3.00 2.63 0.002 0.021 0.0037 1.69Tr. Tr. COMPARATIVE EXAMPLE D 0.002 3.00 2.65 0.002 0.007 0.0044 1.680.05 Tr. EXAMPLE E 0.002 3.12 3.23 0.002 0.016 0.0040 1.51 Tr. 0.06EXAMPLE F 0.003 3.20 3.30 0.002 0.014 0.0036 1.55 0.12 Tr. EXAMPLE G0.003 2.99 2.62 0.002 0.020 0.0034 1.68 0.05 Tr. COMPARATIVE EXAMPLE※The underlined value indicates out of the range of the presentinvention.

The specimens with a square 55 mm on a side were punched, the stressrelief annealing was conducted at 750° C. for 2 hours, and then, themagnetic properties (magnetic flux density B8 and core loss W10/400)along L-direction and C-direction were evaluated by the single sheettester (SST) method. The steel sheet in which the magnetic flux densityB8 along L-direction was 1.60 to 1.77 T was judged to be acceptable, thesteel sheet in which the core loss W10/400 along L-direction was 13.0W/kg or less was judged to be acceptable, and the steel sheet in whichthe average of the core losses W10/400 along L-direction and C-directionwas 14.5 W/kg or less was judged to be acceptable. Also, for thecomparison with the conventional grain-oriented electrical steel sheet,the magnetic properties of commercial steel sheet of JIS standard 30P105grade were evaluated as well.

The results are shown in the Table 2.

TABLE 2 CHEMICAL COMPOSITION OF STEEL SLAB (mass %) B8 (T) W10/400(W/kg) Si − L- L- C- L&C STEEL 0.5 × DIREC- DIREC- DIREC- AVER- No. TYPEC Si Mn S sol. Al Mn Sn Sb TION TION TION AGE NOTE 1 A 0.001 3.00 2.630.002 0.001 1.69 Tr. Tr. 1.52 14.0 15.5 14.8 COMPARATIVE EXAMPLE 2 B0.002 2.99 2.62 0.002 0.007 1.68 0.03 Tr. 1.70 10.4 16.0 13.2 EXAMPLE 3C 0.002 3.00 2.63 0.002 0.020 1.69 Tr. Tr. 1.48 17.1 17.8 17.5COMPARATIVE EXAMPLE 4 D 0.001 3.00 2.65 0.002 0.005 1.68 0.05 Tr. 1.709.5 15.9 12.7 EXAMPLE 5 E 0.001 3.12 3.23 0.002 0.016 1.51 Tr. 0.06 1.689.3 15.6 12.5 EXAMPLE 6 F 0.002 3.20 3.30 0.002 0.013 1.55 0.12 Tr. 1.689.2 15.5 12.4 EXAMPLE 7 G 0.002 2.99 2.62 0.002 0.019 1.68 0.05 Tr. 1.5413.8 15.5 14.7 COMPARATIVE EXAMPLE 8 30P105 Tr. 3.00 Tr. Tr. Tr. ≈3.00  Tr. Tr. 1.93 10.1 28.2 19.2 COMPARATIVE EXAMPLE ※The underlined valueindicates out of the range of the present invention.

As shown in Table 2, in the examples of the steel types B, D, E, and F(No.2, 4, 5, and 6) which were the materials including sol. Al, sincethe secondary recrystallization occurred in the entire surface ofspecimen, the L-direction core loss W10/400 and the L&C average coreloss W10/400 were acceptable. On the other hand, in the steel type A(No.1) where the sol. Al content was less than the lower limit and thesteel types C and G (No.3 and 7) where the sol. Al content was more thanthe upper limit, since the secondary recrystallization did not occursufficiently, the magnetic flux density B8 was insufficient, and theL-direction core loss W10/400 and the L&C average core loss W10/400 wereinsufficient.

In addition, in comparison between the core loss W10/400 of the examplesof the steel types B, D, E, and F in (No.2, 4, 5, and 6) and the coreloss W10/400 of the commercial steel sheet (No.8) of JIS standard 30P105grade, although the L-direction core losses thereof were substantiallythe same, the L&C average core losses of the examples were significantlyimproved.

Example 2

The steel slabs whose chemical compositions were shown in Table 3 withthe balance consisting of Fe and impurities were heated to 1200° C. andthen were hot-rolled so that the thickness was 2.1 mm. The hot-bandannealing was conducted at 900° C. for 30 seconds, the cold-rolling wasconducted so that the final thickness was 0.35 mm, the primaryrecrystallization annealing was conducted at 920° C. for 15 seconds, theannealing separator was applied, the secondary recrystallizationannealing was conducted at the maximum temperature of 940° C., andthereafter, the insulation coating was formed. In addition, in order toevaluate the effect of the thickness of product, the steel sheet wherethe cold-rolling was conducted so that the final thickness was 0.50 mmwas produced, wherein the production conditions except for the finalthickness were the same.

In the primary recrystallization annealing process, the heating rate inthe temperature elevating stage was 200° C./second, the atmosphere inthe temperature elevating stage was 25% of H2 and 75% of N2, theatmosphere in the holding stage was 25% of H2 and 75% of N2, and the dewpoint in the holding stage was −20° C. The alumina-based separator wasused for the annealing separator. In the secondary recrystallizationannealing process, the heating rate in the temperature elevating stageof 800° C. or higher was 15° C./hour, the temperature was elevated to940° C. by the constant heating rate in substance, the atmosphere in thetemperature elevating stage was 90% of H2 and 10% of N2, the dew pointin the temperature range of 500° C. or higher in the temperatureelevating stage was −30° C., the time in the holding stage was 10 hours,the atmosphere in the holding stage was 100% of H2, and the dew point inthe holding stage was −40° C. In all steel sheets, the insulationcoating was arranged in directly contact with the steel layer, and theadhesion was sufficient.

TABLE 3 CHEMICAL COMPOSITION OF STEEL SLAB (mass %) Si − STEEL 0.5 ×TYPE C Si Mn S sol. Al N Mn Sn Sb NOTE H 0.003 3.03 2.80 0.003 0.0130.0047 1.63 0.05 Tr. EXAMPLE I 0.003 3.05 2.80 0.003 0.012 0.0045 1.650.08 Tr. EXAMPLE J 0.003 3.10 2.82 0.002 0.011 0.0048 1.69 Tr. 0.05EXAMPLE K 0.002 3.40 3.03 0.002 0.015 0.0050 1.89 0.06 Tr. EXAMPLE L0.002 3.38 2.58 0.002 0.015 0.0053 2.09 0.05 Tr. COMPARATIVE EXAMPLE M0.003 3.06 1.47 0.003 0.011 0.0045 2.33 0.05 Tr. COMPARATIVE EXAMPLE※The underlined value indicates out of the range of the presentinvention.

The specimens with a square 55 mm on a side were punched, the stressrelief annealing was conducted at 750° C. for 2 hours, and then, themagnetic properties (magnetic flux density B8 and core loss W10/400)along L-direction and C-direction were evaluated by the single sheettester (SST) method. The steel sheet in which the magnetic flux densityB8 along L-direction was 1.60 to 1.77 T was judged to be acceptable, thesteel sheet in which the core loss W10/400 along L-direction was 13.0W/kg or less was judged to be acceptable, and the steel sheet in whichthe average of the core losses W10/400 along L-direction and C-directionwas 14.5W/kg or less was judged to be acceptable. The results are shownin the Table 4.

TABLE 4 CHEMICAL COMPOSITION OF STEEL SLAB (mass %) B8 (T) W10/400(W/kg) Si − L- L- C- L&C STEEL 0.5 × THICKNESS DIREC- DIREC- DIREC-AVER- No. TYPE C Si Mn S sol. Al Mn Sn Sb (mm) TION TION TION AGE NOTE 9H 0.002 3.03 2.80 0.003 0.012 1.63 0.05 Tr. 0.35 1.69 12.0 16.6 14.3EXAMPLE 10 I 0.002 3.05 2.80 0.003 0.011 1.65 0.08 Tr. 0.35 1.71 10.916.9 13.9 EXAMPLE 11 I 0.002 3.05 2.80 0.003 0.011 1.65 0.08 Tr. 0.501.72 16.9 25.0 21.0 REFERENCE EXAMPLE 12 J 0.002 3.10 2.82 0.002 0.0101.69 Tr. 0.05 0.35 1.70 11.0 16.9 14.0 EXAMPLE 13 K 0.002 3.40 3.030.002 0.015 1.89 0.06 Tr. 0.35 1.69 10.5 16.5 13.5 EXAMPLE 14 L 0.0023.38 2.58 0.002 0.015 2.09 0.05 Tr. 0.35 1.57 14.1 17.2 15.7 COMPARATIVEEXAMPLE 15 M 0.002 3.06 1.47 0.003 0.010 2.33 0.05 Tr. 0.35 1.59 13.917.4 15.7 COMPARATIVE EXAMPLE ※The underlined value indicates out of therange of the present invention.

As shown in Table 4, in the examples of the steel types H, I, J, and K(No. 9, 10, 12, and 13) which were the materials with the thickness of0.35 mm, since the secondary recrystallization occurred in the entiresurface of specimen, the L-direction core loss W10/400 and the L&Caverage core loss W10/400 were acceptable. On the other hand, in thesteel type I (No.11) where the thickness was 0.5 mm and was thicker thanthe upper limit, the L-direction core loss W10/400 and the L&C averagecore loss W10/400 were significantly insufficient. Also, in the steeltypes L and M (No. 14 and 15) where the value of “Si−0.5×Mn” was morethan the upper limit, snice the linear defect of the secondaryrecrystallization occurred in many areas, the magnetic flux density B8was insufficient, and the L-direction core loss W10/400 and the L&Caverage core loss W10/400 were insufficient.

Example 3

The steel slabs whose chemical compositions were shown in Table 5 withthe balance consisting of Fe and impurities were heated to 1250° C. andthen were hot-rolled so that the thickness was 2.8 mm. The firstcold-rolling was conducted so that the thickness was 1.4 mm, theintermediate annealing was conducted at 950° C. for 30 seconds, thesecond cold-rolling was conducted so that the final thickness was 0.23mm, the primary recrystallization annealing was conducted at 920° C. for15 seconds, the annealing separator was applied, the secondaryrecrystallization annealing was conducted at the maximum temperature of940° C., and thereafter, the insulation coating was formed.

In the primary recrystallization annealing process, the heating rate inthe temperature elevating stage was 1000° C./second, the atmosphere inthe temperature elevating stage was 15% of H2 and 85% of N2, theatmosphere in the holding stage was 15% of H2 and 85% of N2, and the dewpoint in the holding stage was −30° C. The alumina-based separator wasused for the annealing separator. In the secondary recrystallizationannealing process, the heating rate in the temperature elevating stageof 800° C. or higher was 20° C./hour, the temperature was elevated to940° C. by the constant heating rate in substance, the atmosphere in thetemperature elevating stage was 95% of H2 and 5% of N2, the dew point inthe temperature range of 500° C. or higher in the temperature elevatingstage was −20° C., the time in the holding stage was 15 hours, theatmosphere in the holding stage was 100% of H2, and the dew point in theholding stage was −40° C. In all steel sheets, the insulation coatingwas arranged in directly contact with the steel layer, and the adhesionwas sufficient.

TABLE 5 CHEMICAL COMPOSITION OF STEEL SLAB (mass %) Si − STEEL 0.5 ×TYPE C Si Mn S sol. Al N Mn Sn Sb NOTE N 0.002 2.99 2.72 0.001 0.0140.0052 1.63 0.05 Tr. EXAMPLE O 0.002 3.00 2.73 0.001 0.015 0.0051 1.640.10 Tr. EXAMPLE P 0.003 3.05 2.68 0.001 0.013 0.0051 1.71 0.03 0.03EXAMPLE ※The underlined value indicates out of ths range of the presentinvention.

The specimens with a square 55 mm on a side were punched, the stressrelief annealing was conducted at 750° C. for 2 hours, and then, themagnetic properties (magnetic flux density B8 and core loss W10/400)along L-direction and C-direction were evaluated by the single sheettester (SST) method. The steel sheet in which the magnetic flux densityB8 along L-direction was 1.60 to 1.77 T was judged to be acceptable, thesteel sheet in which the core loss W10/400 along L-direction was 13.0W/kg or less was judged to be acceptable, and the steel sheet in whichthe average of the core losses W10/400 along L-direction and C-directionwas 14.5 W/kg or less was judged to be acceptable. Also, for thecomparison with the conventional grain-oriented electrical steel sheet,the magnetic properties of commercial steel sheet of JIS standard 23P95grade were evaluated as well.

The results are shown in the Table 6.

TABLE 6 CHEMICAL COMPOSITION OF STEEL SLAB (mass %) B8 (T) W10/400(W/kg) Si − L- L- C- L&C STEEL 0.5 × DIREC- DIREC- DIREC- AVER- No. TYPEC Si Mn S sol. Al Mn Sn Sb TION TION TION AGE NOTE 16 N 0.002 2.99 2.720.001 0.012 1.63 0.05 Tr. 1.63 8.7 13.0 10.9 EXAMPLE 17 O 0.002 3.002.73 0.001 0.013 1.64 0.10 Tr. 1.73 7.0 13.5 10.3 EXAMPLE 18 P 0.0033.05 2.68 0.001 0.012 1.71 0.03 0.03 1.72 7.1 13.4 10.3 EXAMPLE 19 23P95Tr. 3.00 Tr. Tr. Tr. ≈3.00   Tr. Tr. 1.92 7.4 25.1 16.3 COMPARATIVEEXAMPLE ※The underlined value indicates out of the range of the presentinvention.

As shown in Table 6, in the examples of the steel types N, O, and P(No.16, 17, and 18), since the secondary recrystallization occurred, theL-direction core loss W10/400 and the L&C average core loss W10/400 wereacceptable. Also, in comparison between the core loss W10/400 of theexamples of the steel types N, O, and P (No.16, 17, and 18) and the coreloss W10/400 of the commercial steel sheet (No.19) of JIS standard 23P95grade, although the L-direction core losses thereof were substantiallythe same, the L&C average core losses of the examples were significantlyimproved.

Example 4

The steel slabs whose chemical compositions were shown in Table 7 withthe balance consisting of Fe and impurities were heated to 1230° C. andthen were hot-rolled so that the thickness was 2.0 mm. The hot-bandannealing was conducted at 920° C. for 30 seconds, the cold-rolling wasconducted so that the final thickness was 0.30 mm, the primaryrecrystallization annealing was conducted at 930° C. for 15 seconds, theannealing separator was applied, the secondary recrystallizationannealing was conducted at the maximum temperature of 940° C., andthereafter, the insulation coating was formed.

In the primary recrystallization annealing process, the heating rate inthe temperature elevating stage was 120° C./second, the atmosphere inthe temperature elevating stage was 20% of H2 and 80% of N2, theatmosphere in the holding stage was 20% of H2 and 80% of N2, and the dewpoints in the holding stage were under conditions of −25° C., −10° C.,0° C., and 30° C. The alumina-based separator was used for the annealingseparator.

In the secondary recrystallization annealing process, the heating ratein the temperature elevating stage of 800° C. or higher was 20° C./hour,the temperature was elevated to 940° C. by the constant heating rate insubstance, the atmosphere in the temperature elevating stage was 85% ofH2 and 15% of N2, the dew point in the temperature range of 500° C. orhigher in the temperature elevating stage was 0° C., the time in theholding stage was 5 hours, the atmosphere in the holding stage was 100%of H2, and the dew point in the holding stage was −30° C. In the steelsheets of test Nos. 20 and 21, the insulation coating was arranged indirectly contact with the steel layer, and the adhesion was sufficient.On the other hand, in the steel sheets of test Nos. 22 and 23, the oxidewas formed between the insulation coating and the steel layer, and theadhesion was insufficient.

TABLE 7 CHEMICAL COMPOSITION OF STEEL SLAB (mass %) Si − STEEL 0.5 ×TYPE C Si Mn S sol. Al N Mn Sn Sb NOTE Q 0.003 3.10 2.81 0.001 0.0070.0042 1.70 0.030 Tr. EXAMPLE R 0.003 3.00 2.84 0.003 0.015 0.0049 1.580.003 0.0008 COMPARATIVE EXAMPLE S 0.003 2.98 1.97 0.003 0.014 0.00492.00 0.050 0.050 COMPARATIVE EXAMPLE T 0.003 3.25 4.10 0.003 0.0150.0049 1.20 0.050 0.050 COMPARATIVE EXAMPLE ※The underlined valueindicates out of the range of the present invention.

The specimens with a square 55 mm on a side were punched, the stressrelief annealing was conducted at 750° C. for 2 hours, and then, themagnetic properties (magnetic flux density B8 and core loss W10/400)along L-direction and C-direction were evaluated by the single sheettester (SST) method. The steel sheet in which the magnetic flux densityB8 along L-direction was 1.60 to 1.77 T was judged to be acceptable, thesteel sheet in which the core loss W10/400 along L-direction was 13.0W/kg or less was judged to be acceptable, and the steel sheet in whichthe average of the core losses W10/400 along L-direction and C-directionwas 14.5 W/kg or less was judged to be acceptable. The results are shownin the Table 8.

TABLE 8 DEW POINT IN HOLDING STAGE FOR PRIMARY CHEMICAL COMPOSITION OFSTEEL SLAB (mass %) B8 (T) W10/400 (W/kg) RECRYS- Si − L- L- C- L&CSTEEL TALLIZA- 0.5 × DIREC- DIREC- DIREC- AVER- No. TYPE TION C Si Mn Ssol. Al Mn Sn Sb TION TION TION AGE NOTE 20 Q −25° C. 0.002 3.10 2.810.001 0.006 1.70 0.03 Tr. 1.71 9.5 15.7 12.6 EXAMPLE 21 Q −10° C. 0.0023.10 2.81 0.001 0.005 1.70 0.03 Tr. 1.70 9.8 15.6 12.7 EXAMPLE 22 Q  0°C. — — — — — — — — — — — — COMPARATIVE EXAMPLE 23 Q  30° C. — — — — — —— — — — — — COMPARATIVE EXAMPLE 24 R −25° C. 0.002 3.00 2.84 0.003 0.0141.58 0.003 0.0008 1.69 13.1 16.0 14.6 COMPARATIVE EXAMPLE 25 S −25° C.0.003 2.98 1.97 0.003 0.013 2.00 0.050 0.050 1.68 13.2 15.9 14.6COMPARATIVE EXAMPLE 26 T −25° C. 0.003 3.25 4.10 0.003 0.014 1.20 0.0500.050 1.57 14.1 17.2 15.7 COMPARATIVE EXAMPLE ※The underlined valueindicates out of the range of the present invention.

As shown in Table 8, in the examples of test Nos.20 and 21, since thesecondary recrystallization occurred in the entire surface of specimen,the L-direction core loss W10/400 and the L&C average core loss W10/400were acceptable. Also, in the examples of test Nos.20 and 21, the Ocontent of the steel layer was 0.03% (300 ppm) or less measured by thenon-dispersive infrared absorption method after fusion in a current ofinert gas.

On the other hand, in the comparative examples of test Nos.22 and 23,the O content of the steel layer was more than 0.03% (300 ppm), thethick oxide layer was formed on the surface of steel layer, the adhesionof the insulation coating significantly deteriorated, and thus, themagnetic properties could not be evaluated. Also, in the comparativeexamples of the steel types R, S, and T (No.24 to 26), the magnetic fluxdensity B8 and the core loss W10/400 were insufficient.

INDUSTRIAL APPLICABILITY

According to the above aspects of the present invention, it is possibleto provide the grain-oriented electrical steel sheet excellent in boththe high-frequency magnetic properties in L-direction and the average ofhigh-frequency magnetic properties in L-direction and C-direction.Accordingly, the present invention has significant industrialapplicability.

REFERENCE SIGNS LIST

1: STEEL LAYER (BASE STEEL)

2: INSULATION COATING

3: GLASS FILM (FORSTERITE FILM)

The invention claimed is:
 1. A method of producing a grain-orientedelectrical steel sheet, the method consisting of a casting process, ahot-rolling process, a cold-rolling process, a primary recrystallizationannealing process, an annealing separator coating process, a secondaryrecrystallization annealing process, and an insulation coating formationprocess, wherein: in the casting process, a steel is cast so that thesteel includes, as a chemical composition, by mass %, 0.0003 to 0.005%of C, 2.9 to 4.0% of Si, 2.0 to 4.0% of Mn, 0.003 to 0.018% of sol. Al,0.001 to 0.01% of N, 0.005% or less of S, 0 to 0.20% of Sn, 0 to 0.20%of Sb, and a balance consisting of Fe and impurities, a silicon contentand a manganese content expressed in mass % in the chemical compositionsatisfy 1.2% <Si - 0.5 x Mn <2.0%, and a tin content and an antimonycontent expressed in mass % in the chemical composition satisfy0.005%≤Sn+Sb≤0.20%; in the primary recrystallization annealing process,a primary recrystallization annealing is conducted for the steel underconditions such that a heating rate in a temperature elevating stage is100° C./second to 5000° C./second on average, an atmosphere in thetemperature elevating stage is 10 to 100 vol % of H2 and H2+N2=100 vol%, a temperature in a holding stage is 800 to 1000° C., a time in theholding stage is 5 seconds to 10 minutes, an atmosphere in the holdingstage is 10 to 100 vol % of H2, H2+N2=100 vol %, and a dew point is −10°C. or lower; in the annealing separator coating process, an annealingseparator including an alumina as a main component is only applied onthe steel; and in the secondary recrystallization annealing process, asecondary recrystallization annealing is conducted for the steel underconditions such that an atmosphere in a temperature elevating stage is 0to 80 vol % of N2 and H2+N2=100 vol %, a dew point in a temperaturerange of 500° C. or higher in the temperature elevating stage is 0° C.or lower, a temperature in a holding stage is 850 to 1000° C., a time inthe holding stage is 4 to 100 hours, an atmosphere in the holding stageis 0 to 80 vol % of N2, H2+N2=100 vol %, and a dew point is 0° C. orlower; and in the insulation coating formation process an insulationcoating is formed on the steel after the secondary recrystallizationannealing process.
 2. The method of producing a grain-orientedelectrical steel sheet according to claim 1, wherein, in the castingprocess, the steel includes, as the chemical composition, by mass %,0.004 to 0.20% of Sn, and 0.001 to 0.20% of Sb.
 3. The method ofproducing a grain-oriented electrical steel sheet according to claim 1,wherein, in the secondary recrystallization annealing process, the steelis heated to the temperature in the holding stage by a constant heatingrate in the temperature elevating stage.
 4. The method of producing agrain-oriented electrical steel sheet according to claim 2, wherein, inthe secondary recrystallization annealing process, the steel is heatedto the temperature in the holding stage by a constant heating rate inthe temperature elevating stage.
 5. A method of producing agrain-oriented electrical steel sheet, the method consisting of acasting process, a hot-rolling process, a cold-rolling process, aprimary recrystallization annealing process, an annealing separatorcoating process, a secondary recrystallization annealing process, and aninsulation coating formation process, wherein: in the casting process, asteel is cast so that the steel includes, as a chemical composition, bymass %, 0.0003 to 0.005% of C, 2.9 to 4.0% of Si, 2.0 to 4.0% of Mn,0.003 to 0.018% of sol. Al, 0.001 to 0.01% of N, 0.005% or less of S, 0to 0.20% of Sn, 0 to 0.20% of Sb, and a balance including Fe andimpurities, a silicon content and a manganese content expressed in mass% in the chemical composition satisfy 1.2%≤Si−0.5×Mn≤2.0%, and a tincontent and an antimony content expressed in mass % in the chemicalcomposition satisfy 0.005%≤Sn+Sb≤0.20%; in the primary recrystallizationannealing process, a primary recrystallization annealing is conductedfor the steel under conditions such that a heating rate in a temperatureelevating stage is 100° C./second to 5000° C./second on average, anatmosphere in the temperature elevating stage is 10 to 100 vol % of H2and H2+N2=100 vol %, a temperature in a holding stage is 800 to 1000°C., a time in the holding stage is 5 seconds to 10 minutes, anatmosphere in the holding stage is 10 to 100 vol % of H2, H2+N2=100 vol%, and a dew point is −10° C. or lower; in the annealing separatorcoating process, an annealing separator including an alumina as a maincomponent is only applied on the steel; and in the secondaryrecrystallization annealing process, a secondary recrystallizationannealing is conducted for the steel under conditions such that anatmosphere in a temperature elevating stage is 0 to 80 vol % of N2 andH2+N2=100 vol %, a dew point in a temperature range of 500° C. or higherin the temperature elevating stage is 0° C. or lower, a temperature in aholding stage is 850 to 1000° C., a time in the holding stage is 4 to100 hours, an atmosphere in the holding stage is 0 to 80 vol % of N2,H2+N2=100 vol %, and a dew point is 0° C. or lower; and in theinsulation coating formation process an insulation coating is formed onthe steel after the secondary recrystallization annealing process.